High voltage switching circuit



Nov. 27, 1962 H. B. CODY 3,066,229

HIGH VOLTAGE SWITCHING CIRCUIT Filed May 2, 1958 a 1 i r 7 ZENER 5 1; 12 LOW VOLTAGE MULTIVIBRATOI? m SUPPLY HIGH NEGATIVE VOLMGESOUQCE INVENTOR.

HENRY B. Couv BY @14/% flaw, 244) United States atent Bfihhifil Patented Nov. 27, 1952 hfiee 3,066,229 HIGH VOLTAGE SWITCHING @HRCUTT Henry B. Cody, San Diego, Calif., assignor to General Dynamics (Imp-oration, Rochester, N.Y., a corporation of Delaware Filed May 2, 1958, er. No. 732,e'58 5 Claims. (El. 307--88.5)

The present invention relates to a high voltage switching circuit, and more particularly to a high voltage switching circuit having circuitry in the grid circuit of an e tron tube capable of rapidly changing the bias on the tubes control grid during a predetermined time interval. This change in bias causes the electron tube to rapidly produce a load current in a load resistor creating a large, subtantially static, switching voltage in an output circuit.

An important use of the present invention is in connec tion with a shaped beam cathode ray tube, where it is often desirable to provide for rapid control of its focusing lens. An example of such a need is illustrated in application Serial No. 711,283 filed January 27, 1958, now abandoned, by Hafiinger et al. and assigned to the same assignee as the present application. The lens control unit in the aforesaid application is required to rapidly switch high voltage inputs to the electronic focus lens. This rapid switching of potential to the focus lens permits radar or television information and character images to be simultaneously displayed on the screen of the cathode ray tube. In order to obtain flickerless display of both types of information, rapid switching of large potentials is necessary. In some instances, the potential changes required are in the neighborhood of 4000 volts. Generally, an electrostatic focu lens comprises two electrodes that are spaced from each other, one of which is connected to a constant magnitude potential source and the other electrode being connected to a variable potential source. The focal length of the electronic lens is governed by the potential differential between the two electrodes. Thus, to rapidly change the focal length of the focus lens thereby alternately obtaining a spot size electron beam for radar display and a defused beam for character writing as in the aforesaid application filed by Hafiinger et al., the voltage change on the one electrode is accomplished by a switching circuit. It was in light of the need of a simple switching circuit capable of accomplishing the above purpose that the present invention was conceived.

The invention provides for switching of substantially static high voltages that are applied to an external circuit such as a shaped beam tubes electrostatic focus lens permitting time sharing in the display of the cathode ray tube in the manner aforesaid. The switching circuit is easily controlled through a multivibrator circuit and may be either intermittently operated or selectively operated at desired intervals by triggering pulses. The circuit is capable of switching at a speed that permits an output voltage of substantially a square wave with a duration of only 100 microseconds.

It is therefore an object of this invention to provide a switching circuit capable of rapidly switching a large, substantially static voltage output giving a square wave output of short duration.

It is another object of this invention to provide a simplified switching circuit capable of rapidly switching output voltages between two predetermined voltage levels.

Other objects and features of this invention will be readily apparent to those skilled in the art from the following specification and appended drawings illustrating certain preferred embodiments of this invention, in which:

The single sheet of drawings illustrates schematically an embodiment of the circuit arrangement and components in the high speed switching circuit as used with a shaped beam cathode ray tube.

In accordance with the specific embodiment of this invention, the switching circuit alternately switches static voltage conditions on the electrostatic focus lens 24 of cathode ray tube 21 between substantially a ground potential and a large negative voltage. Under one mode of operation, the voltage in line 20 may be switched from substantially ground potential to approximately 4000 volts of negative potential.

Electron tube 7 has its cathode 13 connected to its control electrode or grid 14 through line 18. A high negative voltage source 22 maintains the cathode to grid circuit at a high negative potential. In the aforesaid mode of operation, this negative voltage would be approximately 5000 volts. The potential source to anode or plate 15 of tube 7 is that from ground 23 through resistor 19. Since the plate circuit of tube 7 in the specific embodiment is connected to ground, the tubes operation is between a high negative potential and ground. The magnitude of the voltage dilferential across the tube may be used to control the magnitude of the voltage change resulting from the switching operation. Thus, the voltage across the tube may be selectively varied and thus, vary the magnitude of the switching voltage.

Under static or non-switching conditions, tube 7 is biased to cut off by the voltage drop across unidirectional device 12. Low voltage supply 8 is connected in series with uni-directional 12 and causes a current flow therethrough. Resistor l7 limits this current flow in the series circuit to a predetermined magnitude. Uni-directional device 12 conducts only in a direction toward the cathode 13 and provides a constant voltage bias between cathode l3 and grid 14 that is relatively independent of the low voltage supply, thus, it functions somewhat as a voltage regulator in that it is not responsive to minor fluctuations in the voltage output of the low voltage supply 8. The grid bias as seen across uni-directional device 12 in the specific embodiment is approximately 22 volts, which is sufficient to bias tube 7 to a cut off condition.

Transistor 6 has a base, a collector 11 and an emitter 10. It is selectively energized and de-energized by a multivibrator circuit 5. In the initial stable state, transistor 6 is non-conducting. Its collector to emitter circuit, which is an alternate path from cathode 13 to grid 14, may be considered to be an open circuit. In the switching or operating phase, a trigger pulse is supplied to the input line 3 through coupling capacitor 4- to the multivibrator circuit 5. The multivibrator circuit may be any of the types known in the art utilizing transistors, electron tubes or the like. If it is desired to have the switching circuit operate continuously for a predetermined time, then a free-running multivibrator may be utilized with an added circuit applied thereto for shutting off the multivibrator at controlled intervals. In this mode of operation, the time interval for each switching operation is controlled by the multivibrator. The multivibrator may 3 also 'be of the stable state type as disclosed in the specific embodiment wherein the triggering pulse from input line 3 can initially set the stable state multivibrator into an operating condition and then subsequently place it in an inoperative condition.

When triggered, multivibrator 5 provides a positive pulse to the base of NPN transistor 6 through line 9. The collector ll of transistor 6 receives positive potential from low voltage supply 8 and the emitter it} of transistor 6 is negative from the same source thus, when receiving a positive pulse through line 9, the transistor conducts. The flow of current through transistor s when conducting, causes the voltage drop across the transistor to decrease to a point that its collector to emitter circuit becomes substantially a short circuit. The ratio of the initial voltage drop across the collector to emitter circuit to that when transistor 6 is conducting, may be as high as 100 to 1.

The conducting of transistor 6 effectively short circuits the potential drop across uni-directional device 12, causing grid 14 and the cathode 13 of tube 7 to be at substantially the same potential. This results in zero bias causing tube 7 to conduct. The increased current therefrom in the plate circuit flows through a large resistor 19 to ground 23. inasmuch as line 2a? is connected to electrostatic electrode 26 that is spaced from electrode 25, line 20 leads to an open circuit and current will not fiow through this circuit. The other electrode 25 of the electrostatic focus lens is connected to substantially the same potential as ground 23 thus, the voltage across the two electrodes is substantially that voltage dro across resistor 19. The conducting of tube 7 in the manner described is capable of dropping the static potential at line 20 from ground potential to a negative potential of approximately 4000 volts, while providing a substantially square wave output pulse for a duration of only 109 microseconds.

Thus, the operation of the switching circuit is as follows. In its first stable state, transistor 6 is an open circuit. The bias voltage on grid 14, as seen across reference uni-directional device 12, biases tube 7 to cut off. In its second state, transistor 6 is turned on by a triggering pulse. The transistor then conducts, causing a short circuit in the cathode to grid circuit of tube 7 which connects cathode 13 or" tube 7 to its own grid 14. With substantially a zero bias, tube 7 conducts, causing plate current to flow through resistor 19 developing a definite voltage drop which is applied to the electrostatic focusing lens of the cathode ray tube.

While certain preferred embodiments of the invention has been specifically disclosed, it is understood that the invention is not limited thereto as other variations may be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

I claim:

1. A high voltage switching circuit comprising a transistor having emitter, collector and base electrodes, a tube including an anode, a cathode and a control electrode, means connecting said collector electrode to said cathode and means connecting said emitter to said control electrode, a voltage-regulating uni-directional device and a low voltage supply, said uni-directional device being connected in parallel with said transistor between said cathode and said control electrode, means for connecting said low voltage supply in parallel with said uni-directional device between said cathode and said control electrode, said low voltage supply causing current to flow through said unidirectional device, a regulated voltage drop appearing across said uni-directional device in response to said current flow for biasing said tube to cutofi, and means for supplying an input switching pulse to said transistor for selectively efifecting conduction of current in said transistor for the duration of said pulse, said transistor When conducting, short circuiting said low voltage supply and I- said unidirectional device and rendering said tube conductmg.

2. A high voltage switching circuit comprising a transistor having an emitter, collector and base electrodes, a tube having an anode, a cathode and a control electrode, means connecting said collector electrode to said cathode and circuit means connecting said emitter to said control electrode, a voltage-regulating uni-directional device connected between said collector and emitter of said transistor and between said cathode and said control electrode or said tube, a low voltage supply and a current providing a voltage drop across said uni-directional device for biasing said control electrode negative relative to said cathode and causing said tube to be biased to cut off condition, a high voltage source for supplying negative voltage to said control electrode, input means for supplying pulses to said base of said transistor for selectively energizing said transistor, said transistor when in the energized condition short circuiting said uni-directional device causing said tube to condut, and anode circuit means connected to said anode for providing a large voltage drop to an external circuit in response to conduction of said tube.

3. A high voltage switching circuit comprising a transistor having an emitter, collector and base electrodes, a tube including an anode, a cathode and a control elec trode, means connecting said collector electrode to said cathode and means connecting said emitter to said control electrode, a voltage regulating uni-directional device, a low voltage supply connected in shunt with said unidirectional device, said uni-directional device being connected between said emitter and said collector, said low voltage supply being connected between said cathode and said control electrode, a voltage drop appearing across said uni-directional device biasing said tube to cutoff, said grid being electrically connected to a source of potential which is highly negative relative to said cathode, means for energizing said transistor, said transistor when energized re ducing said bias on said tube by connecting said cathode to said control electrode causing said tube to conduct a plate circuit connected to said anode having load means across which a relatively large voltage is attained in re sponse to conduction to said tube.

4. A high voltage switching circuit comprising an electron tube having a cathode circuit, a grid circuit and a plate circuit, said electron tube being normally biased to cut off condition by a regulated voltage supply including a unidirectional conducting device, said cathode circuit and said grid circuit being electrically connected to a source of large negative potential, transistor means electrically connected to said cathode circuit and said grid circuit for substantially short circuiting said regulated voltage supply when said transistor means is conducting, means for selectively rendering said transistor means conducting in response to an input switching pulse for causing said tube to conduct for the duration of said pulse, and output means connected in said plate circuit to which said large negative voltage from said cathode circuit is rapidly transferred during selective energization of said transistor means.

5. In combination, an electron tube having an anode, a control grid and a cathode, said grid being electrically connected to a source of potential which is highly negative relative to said cathode, a bias source connected between said cathode and said grid for supplying a constant biasing potential to said tube suflicient to cut off said tube, said bias source including a voltage-regulating Zener diode and a source of reverse current therefor, said diode when reverse biased presenting a high impedance between said source of potential and said cathode, a transistor connected in shunt with said bias source and normally nonconducting, means for selectively supplying a switching pulse to said transistor to render said transistor conducting, said bias source being short-circuited in response to conduction of said transistor during said switching pulse for rendering said tube conductive, and an output circuit connected to said anode, said output circuit providing a high voltage output voltage during conduction of said tube which bears a relation to the potential from said potential source.

References Cited in the file of this patent UNITED STATES PATENTS 6 Shockley Aug. 2, 1955 Lohman et a1. Feb. 28, 1956 Guggi Oct. 8, 1957 Bergfors Mar. 4, 1958 Bateinan Apr. 22, 1958 Merrill Oct. 14, 1958 Koch Dec. 2, 1958 Dickinson Feb. 3, 1959 Scanlon Sept. 29, 1959 

